One example of a prior art hydrodynamic separator is shown in U.S. Pat. No. 5,788,848 patent. This patent shows a system comprising inner and an outer non-concentric cylinders, the inner cylinder including a screen portion (the screened separator). The debris containing stream is feed to the internal space within the inner cylindrical screened separator; the material contained therein is retained within and below the inner cylinder while the fluid flows from the inside of the inner screened cylinder to an annular space between the inner and outer cylinder and exits from the surrounding outer chamber.
U.S. Pat. No. 6,241,881 discloses a similar cylindrical waste separator which includes on its upper portion an inlet for loaded influent and an outlet for cleaned effluent. The separator comprises a cylindrical portion having a lower part incorporating a basket. The inflow stream containing solid matter flows in a rotary motion to the area inside the cylindrical portion above the basket. The solid matter of a size greater than that of the mesh openings in the basket are retained within the cylinder and in the basket at the lower end of the cylinder and the fluid with solid matter smaller than the mesh openings, referred to as cleaned effluent, passes outward through the basket and cylinder walls. Cleaned effluent entering the peripheral area surrounding the basket and cylinder then flows upwards into the outlet pipe.
These arrangement have the disadvantage of clogging as a result of retained waste material, such as plastic bags, bottles, leaves, etc, that can accumulate against the inner walls of the screen causing the swirling, inflowing stream to penetrate the only the upper portion of the screen which, in turn, causes the inflowing stream to flow only through the upper portion of the basket. Bulky waste obstructing the basket walls thus reduces the flow capacity of liquid passing through the basket and the efficiency of separation and the inflowing stream tends to bypass the separator and flow directly to the outlet through an overflow weir.
U.S. Pat. No. 6,641,720, shows a separator which has a plurality of protruding segments adjacent openings in the panel, with each segment extending from the face of the panel at a position upstream of respective openings so as to project into the fluid flow path to form a substantially closed face to the liquid flowing over the screen, the intent being to prevent blockage of the openings in the screen.
An alternative hydrodynamic separator for urban and industrial effluents, shown in EP Published application 2,181,748, incorporated herein in its entirety by reference, includes a tank having a centrally located cylindrical chamber, the cylindrical chamber having a tubular screen made of expanded or perforated metal within the space defined by the cylindrical chamber. In these separators, fluid flowing into the separator circulates in the tank in a space exterior to the cylindrical chamber. Waste is constrained in a cyclone-like vortex that forms in the tank but outside the screened cylinder and moves downwards to the bottom of the tank while cleaned fluid flows through the lateral surface of an expanded metal screen of the cylinder and exits from the bottom of the centrally located screened chamber.
The rotation of the effluent stream in the periphery helps avoid the deposition of waste or particles on the screen so that the screen remains unobstructed.